Mitochondrial (mt) DNA for Paramecium aurelia is a linear molecule replicating via a lariat (circle plus tail) intermediate and terminating in a linear head-to-head dimer length molecule. In addition, mitochondria from Paramecium can be microinjected within the same species or between species. Species 4 can not serve as either donor or recipient in interspecies transfer with species 1, 5 or 7. We have constructed restriction enzyme maps of some of these species and found that species 1, 5, and 7 are essentially completely homologous but species 4 is less than 50% homologous to species 1 mt DNA. The major region of non-homology involves that concerned with initiation of DNA replication. We propose to construct more detailed restriction maps of species 1 and 5 and especially species 4. Particular attention will be paid to the structure of the initiation region since we have obtained evidence suggesting a difference in the composition of the monomer initiation region with the corresponding dimer region. In addition, we will sequence appropriate cloned fragments of these DNA initiation regions. In this manner we hope to obtain information on the replication mechanism of Paramecium mt DNA as well as gain insight on the general mechanism(s) of mitochondrial DNA. Finally, we will commence studies on identifying the gene products of this mitochondrial DNA, especially with regard to those proteins required for DNA replication.